Interior permanent magnet machines are typically constructed with rotors formed of a stack of disk laminations, with each of the laminations having a plurality of pole piece sections connected to one another by circumferential bridges situated at the disk periphery. The pole piece sections and the interior core portion of the laminations define the radial thickness of magnet slots formed in the laminations. Radial ligaments on each disk connect the interior portions of the disk laminations to the circumferential bridges. The disk laminations are stacked so that the magnet slots extend through the stack in an axial direction when the laminations are assembled on a motor shaft. The magnets are placed in the magnet slots and may be held in place by plastically deforming the bridges and ligaments to create a predetermined hoop stress to hold the magnets in their respective slots when the pressing force is removed. Since the bridges are relatively thin in order to minimize the amount of magnetic flux required to saturate them to assure proper operation of the motor, and since the bridges are of the same magnetic material as the rest of the rotor, their tensile strength is low and limits the maximum operating speed of the rotor. One method of increasing the strength of the rotor is to remove a portion of the low strength material of the laminations and replace that material with higher strength non-magnetic segments. These segments may be positioned at the intersection of the bridges and the radial ligaments and may be welded to both the bridges and ligaments. Alternatively, the segments may be welded to only the bridges so as to form an outer ring which may be shrunk fit over the assembled interior portion of the laminations and magnets. Since the inserts may be formed of a higher strength material than the laminations, the strength of each lamination is increased thus increasing the maximum speed capability of the rotor. A description of a system employing such inserts is shown in U.S. Pat. No. 4,916,346 assigned to the assignee of the present invention.
As the development of interior permanent magnet machines has progressed, the need for machines capable of developing higher torque and horsepower has grown. In order to generate the increased horsepower, it is necessary to increase the amount of flux in such machines while at the same time it is desirable to minimize the size of the machines.
For a given magnet energy, there appears to be an upper limit on motor torque and horsepower in a given frame size when magnets of simple configuration are used. The available motor torque and horsepower may be increased by use of flux multiplication techniques. One method is to use circumferential flux magnets, but this is only effective for pole numbers higher than four which, in turn, requires increased drive frequency. Use of circumferential flux magnets, however, is not always feasible due to electronic device limitations. Flux multiplication may be achieved in low pole number motors by folding the magnet, and such schemes have been proposed, but it is believed that they have not been implemented due to their mechanical complexity. In addition, prior development of folded magnet rotors has been directed at small (25 HP and under) motors which undergo relatively low mechanical stress and thus allow simple implementations. Current interest is now in the 300 HP and higher range, at high mechanical stress.
Folded magnets have been used in the stator poles of low horsepower direct current (DC) motors. U.S. Pat. No. 3,840,763, assigned to the assignee of the present invention, describes the use of folded magnets for alternating current (AC) motors and discloses a proposed V-shaped magnet arrangement and a two-pole, spiral pole arrangement having low stress. U.S. Pat. No. 4,388,545, also assigned to the assignee of the present invention, describes various "cup" and circumferential configurations intended to increase the magnetic flux for application in small motors.